Update the Gemfile to use the `github-pages` gem.

This is the advised way to run GitHub pages projects locally with the same dependencies as the GitHub servers. (I don't know if anyone is trying it locally when writing blog posts, but it would a nice sanity check :smile:)

This does a major update on Jekyll, but as far as I can tell the latest posts all look the same and does not seems to be anything broken with the update.